[cxx-interop] Work around crash in codegen when initializing C++ span

When initializing span with an UnsafePointer<Element>? we call into the
generic initializer that we imported from the C++ templated constructor
instead of the concrete initializer we have in the overlay that takes an
UnsafePointer<Element> (non-optional). We cannot properly codegen for
this generic initializer at the moment, so let's stop importing them
since the user probably wanted to call the initializer from the overlay.

We should come back later and fix the root cause.

rdar://148961349

Author: Gabor Horvath

lib/ClangImporter/ImportDecl.cpp

@@ -4110,6 +4110,15 @@ namespace {
         if (ctordecl->isCopyConstructor() || ctordecl->isMoveConstructor())
           return nullptr;
 
+        // Don't import the generic ctors of std::span, rely on the ctors that
+        // we instantiate when conforming to the overlay. These generic ctors
+        // can cause crashes in codegen.
+        // FIXME: figure out why.
+        const auto *parent = ctordecl->getParent();
+        if (funcTemplate && parent->isInStdNamespace() &&
+            parent->getIdentifier() && parent->getName() == "span")
+          return nullptr;
+
         DeclName ctorName(Impl.SwiftContext, DeclBaseName::createConstructor(),
                           bodyParams);
         result = Impl.createDeclWithClangNode<ConstructorDecl>(

test/Interop/Cxx/stdlib/use-std-span-typechecker.swift

@@ -11,3 +11,11 @@ arr.withUnsafeBufferPointer { ubpointer in
     let _ = ConstSpanOfInt(ubpointer.baseAddress!, ubpointer.count) 
     // expected-warning@-1 {{'init(_:_:)' is deprecated: use 'init(_:)' instead.}}
 }
+
+arr.withUnsafeBufferPointer { ubpointer in 
+    // FIXME: this crashes the compiler once we import span's templated ctors as Swift generics.
+    let _ = ConstSpanOfInt(ubpointer.baseAddress, ubpointer.count)
+    // expected-error@-1 {{value of optional type 'UnsafePointer<Int32>?' must be unwrapped to a value of type 'UnsafePointer<Int32>'}}
+    // expected-note@-2 {{coalesce using '??' to provide a default when the optional value contains 'nil'}}
+    // expected-note@-3 {{force-unwrap using '!' to abort execution if the optional value contains 'nil'}}
+}